This application claims the priority of Japanese patent application Serial No. 11-310064 filed on Oct. 29, 1999.
The present invention relates to a shadow mask, especially, of a low thermal (heat) expansion type to be usable for a color Braun tube of, for example, a color television set or computer, and more particularly, to a shadow mask for a Braun tube capable of being easily press-molded at a time of manufacture and having a small spring-back after the press-molding.
A shadow mask used for a color Braun tube of such as color television or color computer is disposed to a predetermined position in the color Braun tube and formed. with a number of small holes, i.e. apertures, through which electron beams are irradiated to a fluorescent material on an inner surface of the Braun tube. In the Braun tube of this structure, since there is a possibility that a part of the electron beams emitted from an electron gun also collide with the surface of the shadow mask without passing through the apertures, the shadow mask is heat-generated through the collision of the electron beams. However, since a conventional shadow mask is formed by utilizing a low carbon steel plate having a large thermal expansion coefficient, the shadow mask thus formed was liable to cause a thermal expansion due to the heat generation through the collision of the electron beam, and hence, such phenomenon as position shifting of the aperture or shape deformation thereof was easily caused. Such phenomenon causes the position shifting of the electron beam reaching the fluorescent surface in the Braun tube, which results in color shifting of a formed image, thus providing a problem.
Incidentally, it is desired for a shadow mask to have an excellent press-moldability (workability) because the shadow mask must be press-molded in conformity with an inner shape of the Braun tube. However, since an original plate member for the shadow mask formed of the low carbon steel plate mentioned above has provided no excellent press-moldability, in the conventional technology, the shadow mask plate member just before the press-molding was annealed at a temperature of about 700-1000xc2x0 C. under an oxidation-reduction atmosphere so as to improve the press-moldability. However, this annealing process poses problems such that the number of the processes of manufacturing the Braun tube is increased, plate members for the shadow masks adhere to each other during the annealing process and/or the annealing itself is unevenly performed, which result in distortion of the plate members. Such problems lead to the lowering of the manufacturing efficiency of the shadow mask and, moreover, the image display quality of the manufactured Braun tube was also deteriorated.
Furthermore, in prior art, it was intended to solve these problems by using an iron-based alloy plate as a plate member for a shadow mask, containing 30-45 weight % of nickel so as to prescribe the bearing force and expansion thereof within predetermined ranges.
However, in the prior art using such plate member for the shadow mask, a desired press-moldability could not be achieved, and moreover, no countermeasure was taken for preventing the spring back caused during the press molding process.
An object of the present invention is to substantially eliminate defects or drawbacks encountered in the prior art mentioned above and to provide a shadow mask for a Braun tube capable of further improving the press-moldability of a plate member for the shadow mask and suppressing the occurrence of the spring back as much as possible at the time of the press molding.
This and other objects can be achieved according to the present invention by providing a shadow mask for a Braun tube composed of an iron-based alloy plate of a low thermal expansion structure containing nickel and cobalt, wherein the iron-based alloy plate has a crystal grain size of more than 6.0 and less than 9.0 measured by a grain size measuring method prescribed in JIS (Japanese Industrial Standard) G0551, and 0.2% proof stress of more than 240N/mm2 and less than 320N/m2.
In a preferred embodiment, the crystal grain size of said iron-based alloy plate is preferably of more than 7.5 and less than 8.5 and the 0.2% proof stress thereof is preferably of more than 270N/mm2 and less than 300N/mm2. A maximum value of degree of integration of the respective crystal faces of the iron-based alloy plate is of 20%, which is measured by an X-ray diffraction method. The iron-based alloy plate is press-molded to a shadow mask without effecting-annealing process.
More in detail, there is provided a shadow mask for a Braun tube comprising:
a frame member;
a mask body in shape of plate made of an iron-based alloy material; and
an opening portion formed to the mask body,
wherein the iron-based alloy plate of a low thermal expansion structure containing nickel and cobalt, wherein the iron-based alloy plate has a crystal grain size of more than 6.0 and less than 9.0 measured by a grain size measuring method prescribed in JIS (Japanese Industrial Standard) G0551, and 0.2% proof stress of more than 240N/mm2 and less than 320N/mm2.
As mentioned hereinabove, according to the present invention, the thermal expansion is hardly caused even by the generation of heat due to the collision of electron beams and, hence, there is less occur hole position shifting or hole shape deformation. Therefore, any position shifting or deviation of the electron beam reaching the fluorescent surface of the Braun tube is hardly caused, and hence, color shifting or unevenness of an image can be significantly prevented from causing. Furthermore, the shadow mask manufactured by such iron-based allot plate is excellent press-moldability and the generation of the spring back at the time of press-molding can be suppressed, so that an improved manufacturing efficiency of the shadow mask can be achieved, and moreover, the image display quality of the Braun tube using such shadow mask can be further improved.
Furthermore, the anisotropy in the crystal face orientation can be made small, and hence, the anisotropy to the plastic deformability at the time of press-molding can be suppressed as much as possible, thus manufacturing the shadow mask having an excellent dimensional stability, and hence, the image display quality of the Braun tube using such shadow mask can be further improved.
Still furthermore, the number of manufacturing processes or steps can be reduced, so that problems which may be caused in the conventional annealing process will be eliminated. Therefore, the improved manufacturing efficiency of the shadow mask can be achieved, and moreover, the image display quality of the Braun tube using such shadow mask can be further improved.
The nature and further characteristic features of the present invention will be made more clear from the following descriptions made with reference to the accompanying drawings.